Yes, this is just one of those times where we will just have to take context clues from the surrounding text. Which makes an even bigger problem, Duolingo does not provide surrounding context to go by. However, this is how it would go in real-life French conversation.

You need 'ce' in French because 'que' needs to refer to something that is actually mentioned. That's not the case in English, where 'what' can be both the referrer and the referred. So, in French you need 'ce qu'elle disait', and while in English 'what she said' works.

French speakers don't think about the grammar when deciding whether to include ce or not. They just remember that certain kinds of phrases require ce and que together to introduce that phrase.

This is one of those phrases. Practice recognizing those kind of phrases so that you automatically join ce to que to mean what English speakers take as meaning what, as the French do when it is appropriate to include it.

Do not practice thinking of ce que as meaning that which and then converting it to what because if you do that, the thread of the conversation will be long gone while you are doing it.

Yes, but why is "I believe that which they said" incorrect? That is a perfectly logical, and grammatically correct sentence...I know that many people would say "I believe what they said," but the other sentence is fine, even if less utilized.

@kpetersenmd - "I believe that which they said" is somewhere between rather formal and downright stilted. It would have to be an unusual situation where it would be used naturally, even in writing. It's not entirely wrong, per se, but in my opinion it would not be a good translation.

That said, it probably was not accepted because nobody thought to include it in the list of accepted translations. That is, it was an omission rather than a conscious decision to exclude it.

I think that the phrase "to believe in" is more used with people ("I believe in her") or abstract ideas ("I believe in making the most of things"). "what they said" doesn't really fall under those categories in my opinion.

"Ce que" is French for "that which". So, literally translated, this sentence says "I(Je) believe (crois) that which they (ce qu'elles) have said (ont dit)". "I believe they said that" is translated as "Je crois qu'elles l'ont dit." This is where I have a problem. "Disaient" is the imperfect, which is a past progressive tense (They were (verb)-ing) (I used to (verb)). Do you follow? The French translation of "have said" is actually the perfect past (passe compose in French) which denotes an event beginning and ending in the past. Thus, I think the correct translation of this sentence should be, "I believe what they were saying." Any thoughts?

I think how it works is that it was progressive in terms of the time or event. The illustration on another thread was .... we were making the bed today..... could be seen as imperfect if it finished up with ......when a mouse ran across the room..... Making the bed had started in the past and was in still in progress when the event occurred. Unfortunately, Duo seldom establishes that sort of context in these sentences.

Sitesurf has also said the imperfect can be applied when used to fits in the sentence.